In recent years, in an image forming apparatus (for example, MFP), a sheet finishing apparatus is disposed to be adjacent to the latter stage of the MFP in order to perform finishing on a sheet after image formation. The sheet finishing apparatus is called a finisher, and bores a punch hole in the sheet sent from the MFP or staples thereon.
In order to bore the punch hole in the sheet, the finisher is provided with a puncher having a plurality of punching blades. The punching blades are moved up and down by rotation of a punch motor, and the punching blades are moved down toward the paper face of the sheet, so that punch holes are bored in the sheet.
There is a case where the sheet sent from the MFP runs obliquely (hereinafter referred to as skew). When the punch hole is bored while the sheet is skewed, a trouble occurs when filing is performed. Thus, a skew correcting apparatus is provided to correct the skew of the sheet, and the punch hole is bored.
After the punch hole is bored in the sheet, the punching blade is stopped at rising position and the puncher stands by. When the punching blade is separated from the paper face, moves up and is located at the standby position, the position is the home position. Besides, a sensor to detect the lateral edge of the conveyed sheet is attached to the puncher, the puncher is moved in the direction perpendicular to the conveyance direction of the sheet, and the size of the conveyed sheet is detected by the sensor.
According to the sheet size or the hole type of the puncher, when a sheet is conveyed, the sheet may pass over the hole of the puncher, and there is a problem that when a curled sheet or a sheet with a folded front corner is carried in, the front corner of the sheet is caught by the hole of the puncher or the sensor, and a jam occurs.
JP-A-2003-212424 and JP-A-2006-160518 (US2006/0120783A1) disclose a sheet processing apparatus which prevents an end of a sheet from being caught in a hole of a punch unit.
JP-A-2003-267621 discloses a sheet processing apparatus which detects a lateral edge of a sheet and moves the position of a punching unit. JP-A-2001-97638 (U.S. Pat. No. 6,907,806B1) discloses a sheet processing apparatus including a sheet end detection sensor.
However, there is a case where when the sheet size, sheet conveyance speed, or hole type of the puncher varies, the front corner of the sheet is caught in the hole of the puncher, and a further improvement is requested.
Besides, there is a case where a puncher bores a punch hole in a sheet (tab sheet) having a tab. JP-A-2005-47642 discloses a sheet processing apparatus including a unit to determine whether a conveyed sheet is a tab sheet or not. In this example, the paper type is determined, and the stop timing at the penetrate position of the sheet is controlled.
However, there is a defect that since it is difficult to measure a skew amount when the tab sheet is skewed, a punch hole can not be bored at an accurate position.